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<H1 ALIGN="CENTER">Dynamically configured mass virtual hosting</H1>

<P>This document describes how to efficiently serve an arbitrary number
of virtual hosts with Apache 1.3. Some familiarity with
<A HREF="../mod/mod_rewrite.html"><CODE>mod_rewrite</CODE></A> is
useful.</P>

<!--

Written by Tony Finch (fanf@demon.net) (dot@dotat.at).

Some examples were derived from Ralf S. Engleschall's document
	http://www.engelschall.com/pw/apache/rewriteguide/

Some suggestions were made by Brian Behlendorf.

-->

<H2><A NAME="contents">Contents:</A></H2>

<UL>
<LI><A HREF="#motivation">Motivation</A>
<LI><A HREF="#overview">Overview of the technique</A>
<LI><A HREF="#simple">Simple name-based dynamic virtual hosts</A>
<LI><A HREF="#homepages">A virtually hosted homepages system</A>
<LI><A HREF="#xtra-conf">Using a separate virtual host configuration file</A>
<LI><A HREF="#combinations">Using more than one virtual hosting system on the same server</A>
</UL>

<HR><H2><A NAME="motivation">Motivation</A></H2>

<P>The techniques described here are of interest if your
<CODE>httpd.conf</CODE> contains hundreds of
<CODE>&lt;VirtualHost&gt;</CODE> sections that are substantially the
same, for example:
<PRE>
NameVirtualHost 111.22.33.44
&lt;VirtualHost 111.22.33.44&gt;
	ServerName		           www.customer-1.com
	DocumentRoot		/www/hosts/www.customer-1.com/docs
	ScriptAlias  /cgi-bin/  /www/hosts/www.customer-1.com/cgi-bin
&lt;/VirtualHost&gt;
&lt;VirtualHost 111.22.33.44&gt;
	ServerName		           www.customer-2.com
	DocumentRoot		/www/hosts/www.customer-2.com/docs
	ScriptAlias  /cgi-bin/  /www/hosts/www.customer-2.com/cgi-bin
&lt;/VirtualHost&gt;
# blah blah blah
&lt;VirtualHost 111.22.33.44&gt;
	ServerName		           www.customer-N.com
	DocumentRoot		/www/hosts/www.customer-N.com/docs
	ScriptAlias  /cgi-bin/  /www/hosts/www.customer-N.com/cgi-bin
&lt;/VirtualHost&gt;
</PRE>
</P>

<P>The basic idea is to replace all of the static
<CODE>&lt;VirtualHost&gt;</CODE> configuration with a mechanism that
works it out dynamically. This has a number of advantages:
<OL>
    <LI>Your configuration file is smaller so Apache starts faster and
	uses less memory.
    <LI>Adding virtual hosts is simply a matter of creating the
	appropriate directories in the filesystem and entries in the DNS -
	you don't need to reconfigure or restart Apache.
</OL>
</P>

<P>The main disadvantage is that you cannot have a different log file
for each server; however if you have very many virtual hosts then
doing this is dubious anyway because it eats file descriptors. It's
better to log to a pipe or a fifo and arrange for the process at the
other end to distribute the logs (and perhaps accumulate statistics,
etc.). A <CODE>LogFormat</CODE> directive that includes
<CODE>%{SERVER_NAME}e</CODE> for the virtual host makes it easy to do this.</P>


<HR><H2><A NAME="overview">Overview of the technique</A></H2>

<P>All of the dynamic virtual hosts will either be configured as part
of the main server configuration, or within a
<CODE>&lt;VirtualHost&gt;</CODE> section. For a simple (very uniform)
setup, <CODE>&lt;VirtualHost&gt;</CODE> sections aren't needed at all.</P>

<P>A couple of things need to be `faked' to make the dynamic virtual
host look like a normal one. The most important is the server name
(configured with <CODE>ServerName</CODE> and available to CGIs via the
<CODE>SERVER_NAME</CODE> environment variable). The way it is
determined is controlled by the <CODE>UseCanonicalName</CODE>
directive: with <CODE>UseCanonicalName off</CODE> the server name
comes from the contents of the <CODE>Host:</CODE> header in the
request. If there is no <CODE>Host:</CODE> header then the value
configured with <CODE>ServerName</CODE> is used instead.</P>

<P>The other one is the document root (configured with
<CODE>DocumentRoot</CODE> and available to CGIs via the
<CODE>DOCUMENT_ROOT</CODE> environment variable). This is used by the
core module when mapping URIs to filenames, but in the context of
dynamic virtual hosting its value only matters if any CGIs or SSI
documents make use of the <CODE>DOCUMENT_ROOT</CODE> environment
variable. This is an Apache extension to the CGI specification and as
such shouldn't really be relied upon, especially because this
technique breaks it: there isn't currently a way of setting
<CODE>DOCUMENT_ROOT</CODE> dynamically.</P>

<P>The meat of the mechanism works via Apache's URI-to-filename
translation API phase. This is used by a number of modules:
<A HREF="../mod/mod_rewrite.html"><CODE>mod_rewrite</CODE></A>,
<A HREF="../mod/mod_alias.html"><CODE>mod_alias</CODE></A>,
<A HREF="../mod/mod_userdir.html"><CODE>mod_userdir</CODE></A>,
and <A HREF="../mod/core.html">the core module</A>.
In the default configuration these modules are called in that order
and given a chance to say that they know what the filename is. Most of
these modules do it in a fairly simple fashion (e.g. the core module
concatenates the document root and the URI) except for
<CODE>mod_rewrite</CODE>, which provides enough functionality to do
all sorts of sick and twisted things (like dynamic virtual hosting).
Note that because of the order in which the modules are called, using
a <CODE>mod_rewrite</CODE> configuration that matches any URI means
that the other modules (particularly <CODE>mod_alias</CODE>) will
cease to function. The examples below show how to deal with this.</P>

<P><STRONG>The dynamic virtual hosting idea is very simple: use the
server name as well as the URI to determine the corresponding
filename.</STRONG></P>


<HR><H2><A NAME="simple">Simple name-based dynamic virtual hosts</A></H2>

<P>This extract from <CODE>httpd.conf</CODE> implements the virtual
host arrangement outlined in the <A HREF="#motivation">Motivation</A>
section above, but in a generic fashion.</P>

<P>The first half shows some other configuration options that are
needed to make the <CODE>mod_rewrite</CODE> part work as expected; the
second half uses <CODE>mod_rewrite</CODE> to do the actual work. Some
care is taken to do a per-dynamic-virtual-host equivalent of
<CODE>ScriptAlias</CODE>.</P>

<PRE>
# dynamic ServerName
UseCanonicalName Off

# splittable logs
LogFormat "%{SERVER_NAME}e %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon

&lt;Directory /www/hosts&gt;
	# ExecCGI is needed here because we can't force
	# CGI execution in the way that ScriptAlias does
	Options FollowSymLinks ExecCGI
&lt;/Directory&gt;

# now for the hard bit

RewriteEngine On

# a ServerName derived from a Host: header may be any case at all
RewriteMap  lowercase  int:tolower

## deal with normal documents first:
# allow Alias /icons/ to work - repeat for other aliases
RewriteCond  %{REQUEST_URI}  !^/icons/
# allow CGIs to work
RewriteCond  %{REQUEST_URI}  !^/cgi-bin/
# do the magic
RewriteRule  ^/(.*)$  /www/hosts/${lowercase:%{SERVER_NAME}}/docs/$1

## and now deal with CGIs - we have to force a MIME type
RewriteCond  %{REQUEST_URI}  ^/cgi-bin/
RewriteRule  ^/(.*)$  /www/hosts/${lowercase:%{SERVER_NAME}}/cgi-bin/$1  [T=application/x-httpd-cgi]

# that's it!
</PRE>


<HR><H2><A NAME="homepages">A virtually hosted homepages system</A></H2>

<P>This is an adjustment of the above system tailored for an ISP's
homepages server. Using slightly more complicated rewriting rules we
can select substrings of the server name to use in the filename so
that e.g. the documents for <SAMP>www.user.isp.com</SAMP> are found in
<CODE>/home/user/</CODE>. It uses a single <CODE>cgi-bin</CODE>
directory instead of one per virtual host.</P>

<PRE>
RewriteEngine on

RewriteMap   lowercase  int:tolower

# allow CGIs to work
RewriteCond  %{REQUEST_URI}  !^/cgi-bin/

# check the hostname is right so that the RewriteRule works
RewriteCond  ${lowercase:%{HTTP_HOST}}  ^www\.[a-z-]+\.isp\.com$

# concatenate the virtual host name onto the start of the URI
# the [C] means do the next rewrite on the result of this one
RewriteRule  ^(.+)  ${lowercase:%{HTTP_HOST}}$1  [C]

# now create the real file name
RewriteRule  ^www\.([a-z-]+)\.isp\.com/(.*) /home/$1/$2

# define the global CGI directory
ScriptAlias  /cgi-bin/  /www/std-cgi/
</PRE>


<HR><H2><A NAME="xtra-conf">Using a separate virtual host configuration file</A></H2>

<P>This arrangement uses a separate configuration file to specify the
translation from virtual host to document root. This provides more
flexibility but requires more configuration.</P>

<P>The <CODE>vhost.map</CODE> file contains something like this:
<PRE>
www.customer-1.com  /www/customers/1
www.customer-2.com  /www/customers/2
# ...
www.customer-N.com  /www/customers/N
</PRE>
</P>

<P>The <CODE>http.conf</CODE> contains this:
<PRE>
RewriteEngine on

RewriteMap   lowercase  int:tolower

# define the map file
RewriteMap   vhost      txt:/www/conf/vhost.map

# deal with aliases as above
RewriteCond  %{REQUEST_URI}               !^/icons/
RewriteCond  %{REQUEST_URI}               !^/cgi-bin/
RewriteCond  ${lowercase:%{SERVER_NAME}}  ^(.+)$
# this does the file-based remap
RewriteCond  ${vhost:%1}                  ^(/.*)$
RewriteRule  ^/(.*)$                      %1/docs/$1

RewriteCond  %{REQUEST_URI}               ^/cgi-bin/
RewriteCond  ${lowercase:%{SERVER_NAME}}  ^(.+)$
RewriteCond  ${vhost:%1}                  ^(/.*)$
RewriteRule  ^/(.*)$                      %1/cgi-bin/$1
</PRE>
</P>


<HR><H2><A NAME="combinations">Using more than one virtual hosting system on the same server</A></H2>

<P>With more complicated setups, you can use Apache's normal
<CODE>&lt;VirtualHost&gt;</CODE> directives to control the scope of
the various rewrite configurations. For example, you could have one IP
address for homepages customers and another for commercial customers
with the following setup. This can of course be combined with
convential <CODE>&lt;VirtualHost&gt;</CODE> configuration
sections.</P>

<PRE>
UseCanonicalName Off

LogFormat "%{SERVER_NAME}e %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon

&lt;Directory /www/commercial&gt;
	Options FollowSymLinks ExecCGI
	AllowOverride All
&lt;/Directory&gt;

&lt;Directory /www/homepages&gt;
	Options FollowSymLinks
	AllowOverride None
&lt;/Directory&gt;

&lt;VirtualHost 111.22.33.44&gt;
	ServerName www.commercial.isp.com

	RewriteEngine On
	RewriteMap    lowercase  int:tolower

	RewriteCond   %{REQUEST_URI}  !^/icons/
	RewriteCond   %{REQUEST_URI}  !^/cgi-bin/
	RewriteRule   ^/(.*)$  /www/commercial/${lowercase:%{SERVER_NAME}}/docs/$1

	RewriteCond   %{REQUEST_URI}  ^/cgi-bin/
	RewriteRule   ^/(.*)$  /www/commercial/${lowercase:%{SERVER_NAME}}/cgi-bin/$1  [T=application/x-httpd-cgi]
&lt;/VirtualHost&gt;

&lt;VirtualHost 111.22.33.45&gt;
	ServerName www.homepages.isp.com

	RewriteEngine on
	RewriteMap    lowercase  int:tolower

	RewriteCond   %{REQUEST_URI}  !^/cgi-bin/

	RewriteCond   ${lowercase:%{HTTP_HOST}}  ^www\.[a-z-]+\.isp\.com$
	RewriteRule   ^(.+)  ${lowercase:%{HTTP_HOST}}$1  [C]
	RewriteRule   ^www\.([a-z-]+)\.isp\.com/(.*) /www/homepages/$1/$2

	ScriptAlias   /cgi-bin/ /www/std-cgi/
&lt;/VirtualHost&gt;
</PRE>


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<H3 ALIGN="CENTER">
 Apache HTTP Server Version 1.3
</H3>

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